Reflective spectral filtering of high power extreme ultra-violet radiation

ABSTRACT

In one embodiment of the invention, a grating structure etched on a mirror substrate has a grating period causing diffracting, out of an optical path, a first incident radiation within a first band around a first wavelength. A multi-layer coating deposited on the grating structure reflects the first incident radiation, in the optical path, within the first band and a second incident radiation within a second band around a second wavelength. In another embodiment, a first multi-layer coating deposited on a mirror substrate reflects a first incident radiation within a first band around a first wavelength and a second incident radiation, in an optical path, within a second band around a second wavelength. A grating structure is deposited on the first multi-layer coating. The grating structure is etched to have a grating period causing diffracting, out of the optical path, the second incident radiation within the second band.

BACKGROUND

[0001] 1. Field of the Invention

[0002] This invention relates to lithography. In particular, theinvention relates to extreme ultra-violet (EUV) lithography.

[0003] 2. Description of Related Art

[0004] Extreme ultraviolet (EUV) lithography is an imaging technologywith higher resolution capabilities than are available from longerwavelength exposure tools. The very short exposure wavelength requiresan all reflective lens system because refractive materials are notsufficiently transparent. The EUV technique bounces EUV photons off asystem of mirrors, including a mask made of reflective materials, thatis ultimately focused on a resist-coated silicon wafer. By doing so, EUVsystems can pattern features smaller than 0.05 micrometer.

[0005] Present EUV radiation sources have a very broad-band emissionspectrum. For example, Xe laser pulsed plasma (LPP-Xe) sources have inexcess of 40% of their radiation at wavelengths longer than 125nanometers (nm) and approximately 60% of the energy is at wavelengthsthat are longer than 17 nm. EUV lithography tools for high volumemanufacturing may require about 13.4 nm +/−1% wavelength radiation.Power levels are expected to be in the range between 50 and 100 Watts.Thus, kilowatts of energy potentially need to be filtered from thesource spectra. Otherwise, it would unreasonably distort the mask andmirrors used in EUV imaging tools.

[0006] Existing techniques include use of ultra-thin transmissionfilters based on coating of a membrane on support structures. Thistechnique has low efficiency, typically passing only about 50% of thedesired wavelength. In addition, the membrane filters are easy torupture at high power levels because of absorption. Another techniqueuses mono-chrometer and diffraction of the actinic 13.4 nm light. Thistechnique is substantially less efficient and is generally used withonly synchroton sources. Yet, another technique uses cooled Mo/Simulti-layer coated mirrors. This technique operates only throughabsorption and selective reflection, leading to problems in heating anda lack of long wavelength filtering.

[0007] Therefore, there is a need to have an efficient technique forspectral filtering of high power EUV radiation.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] The features and advantages of the present invention will becomeapparent from the following detailed description of the presentinvention in which:

[0009]FIG. 1 is a diagram illustrating a system using a reflectivespectral filter in which one embodiment of the invention can bepracticed.

[0010]FIG. 2 is a diagram illustrating a process to fabricate thereflective spectral filter shown in FIG. 1 according to one embodimentof the invention.

[0011]FIG. 3 is a diagram illustrating a process to fabricate areflective spectral filter shown in FIG. 1 according to anotherembodiment of the invention.

[0012]FIG. 4 is a diagram illustrating reflectance and absorptionspectra for the multi-layer coatings according to another embodiment ofthe invention.

[0013]FIG. 5 is a diagram illustrating a system using a transmittingspectral filter in which one embodiment of the invention can bepracticed.

[0014]FIG. 6 is a diagram illustrating a process to fabricate thetransmitting spectral filter shown in FIG. 5 according to one embodimentof the invention.

DESCRIPTION OF THE INVENTION

[0015] The present invention is a technique for efficient spectralfiltering of EUV radiation. The spectral filter includes a gratingstructure and a multi-layer coating. The multi-layer coating is designedto reflect a narrow band around a short wavelength (e.g., 13.4 nm) andalso reflects long wavelengths (e.g., greater than approximately 60 nm).The grating structure has a grating period responsive to the longwavelength band. The multi-layer coating is formed by layers ofmaterials having high and low atomic numbers (e.g., Mo and Si)interspersed by layers of a compound (e.g., SiC). Regions of the longerwavelength rays are removed from the optical path by diffraction. Theactinic rays have a wavelength much shorter than the grating period andundergoes simple reflection. The combined properties of the multi-layercoating and the grating structure generate the desired spectralcharacteristics.

[0016] In the following description, for purposes of explanation,numerous details are set forth in order to provide a thoroughunderstanding of the present invention. However, it will be apparent toone skilled in the art that these specific details are not required inorder to practice the present invention. In other instances, well-knownstructures are shown in block diagram form in order not to obscure thepresent invention.

[0017]FIG. 1 is a diagram illustrating a system using a reflectivespectral filter in which one embodiment of the invention can bepracticed. The system 100 includes an EUV radiation source 110, acondenser mirror 120, a reflective spectral filter 130, and a baffle140. The system 100 is typically used in EUV lithography.

[0018] The EUV radiation source 110 is a laser pulsed plasma source. AXenon (Xe) gas jet is hit with a high power laser pulse. Ionization andrecombination generates spectral lines ranging from X-ray to Infra-red.As is known by one skilled in the art, any other methods to provide theEUV radiation source can be employed.

[0019] The condenser mirror 120 is an optical subsystem to collect theEUV radiation from the EUV radiation source 110.

[0020] The reflective spectral filter 130 receives the source radiationas provided by the condenser mirror 120 and to reflect actinic rays 160and diffracted rays 150. In one embodiment, the actinic rays areradiation at 13.4 nm and the diffracted rays 150 are at wavelengthslonger than the actinic wavelength (e.g., 60 nm). The reflectivespectral filter 130 will be described in FIGS. 2 and 3.

[0021] The baffle 140 is a metal plate to stop the diffracted rays 150.The baffle 140 has a hole 145 aligned with the principal optical path ofthe spectral filter 130. The actinic rays 160 are on the optical pathand go through the hole 145.

[0022]FIG. 2 is a diagram illustrating a process 200 to fabricate thereflective spectral filter 130 shown in FIG. 1 according to oneembodiment of the invention.

[0023] The process 200 starts with polishing a blank mirror substrate210. The mirror substrate 210 may have a flat or curved surface. Themirror substrate 210 may be made of Zerodur, ULE, or composites with lowcoefficients of thermal expansion that can be polished well. Then, arelief material or a photo-resist layer 220 is deposited on the mirrorsubstrate 210.

[0024] Next, the relief layer 220 is lithographically patterned andetched to form a grating structure 220. The grating structure 220 has aplurality of ridges spaced at a grating period T. It is noted thatalthough the preferred embodiment has a periodic pattern of ridges, itis contemplated that non-periodic pattern may also be used. The ridgeshape is not restricted to the rectangular cross-section shown and couldbe triangular or other possibilities. The grating structure 220 may havea one-dimensional layout or a two-dimensional layout. The layout pattern250 shows a representative two-dimensional layout having ridges 252 andgrooves 254. The grating period T is selected to be responsive to a bandaround a design filter wavelength and selected harmonics. The gratingperiod T is selected to cause diffracting, out of an optical path, anincident radiation within the band of the design filter wavelength. Thedesign filter wavelength may be 60 nm corresponding to when coatingreflectivity begins to increase, 140 nm corresponding to significantemission spectra of the EUV source, or other possibilities. The ridgeshave a ridge width W and height H. The ridge width W is typically lessthan the grating period T. In one embodiment, the ridge width W isapproximately proportionally to the grating period T with aproportionality constant α. In one embodiment, the proportionalityconstant α may be approximately equal to one-half. The ridge height Hmay be any suitable number. In one embodiment, the ridge height H isapproximately proportionally to the grating period T with aproportionality constant β. The proportionality constant β may be an oddinteger number times a quarter of the design filter wavelength. Typicalvalues of the ridge height H may be 35 nm, 105 nm, and 175 nm.

[0025] Then, a multi-layer coating 230 is deposited conformally on thegrating structure 220 and the mirror substrate 210. The multi-layercoating 230 has a number of layers, or stack, of first and secondmaterials having either high and low atomic numbers, respectively, orhigh and low densities of charge carriers, respectively.

[0026] In one embodiment, the first material is molybdenum (Mo) and thesecond material is silicon (Si) or beryllium (Be). The multi-layercoating 230 may also have a number of layers of a compound interspersedwithin the layers of the first and second materials. In one embodiment,the compound is silicon carbide (SiC). The incorporation of the compoundSiC in the stack improves heating durability with minimal reduction(e.g., 3% to 5%) in reflectivity.

[0027] The reflective spectral filter 130 in this embodiment thereforeincludes a multi-layer coating 230, the grating structure 220, and themirror substrate 210. The multi-layer coating 230 is designed to bereflective at around the actinic wavelength (e.g., 13.4 nm) in anoptical path and wavelengths longer than the actinic wavelength (e.g.,60 nm, 140 nm). With this construction, wavelengths that are both nearthe grating period T and that reflect from the multi-layer coating 230are diffracted away (diffracted rays 150 shown in FIG. 1) from the pathof the actinic rays (actinic rays 160 shown in FIG. 1).

[0028]FIG. 3 is a diagram illustrating a process 300 to fabricate thereflective spectral filter 130 shown in FIG. 1 according to anotherembodiment of the invention.

[0029] The process 300 starts with polishing a mirror substrate 310.This step is much similar to the step shown in FIG. 2. Next, a firstmulti-layer coating 320 is deposited on the mirror substrate 310. Thefirst multi-layer coating 320 is made of materials similar to themulti-layer coating 230 in FIG. 2. It includes a number of layers, orstack, of first and second materials having either high and-low atomicnumbers, respectively, or high and low densities of charge carriers,respectively.

[0030] In one embodiment, the first material is molybdenum (Mo) and thesecond material is silicon (Si) or beryllium (Be). The multi-layercoating 230 may also have a number of layers of a compound interspersedwithin the layers of the first and second materials. In one embodiment,the compound is silicon carbide (SiC).

[0031] Then, an etch stop layer 330 is optionally deposited on themulti-layer coating 320. The etch stop layer 330 may be made by SiC orany other suitable material. The etch stop 330 is used so thatsubsequent etching does not cut into the multi-layer coating 320.

[0032] Next, a metal spacer layer 340 is deposited on the etch stoplayer 330 to provide grating relief layer. Then, a second multi-layercoating 350 is deposited on the metal spacer layer 340. This secondmulti-layer coating 350 is essentially the same as the first multi-layercoating 320. Both the multi-layer coatings 350 and 320 are designed tobe reflective at around the actinic wavelength (e.g., 13.4 nm). Notethat the metal spacer layer 340 may not be needed. In addition, thesecond multi-layer coating 350 may not be needed leaving only the metalspacer layer 340.

[0033] The process 300 then lithographically patterns and etches agrating structure 360 from the second multi-layer coating 350 and themetal spacer layer 340. The grating structure 360 may have aone-dimensional layout or a two-dimensional layout. The layout pattern370 shows a representative two-dimensional layout having ridges 372 andgrooves 374. The grating structure 360 has a grating period T responsiveto a band around a longer design filter wavelength (e.g., 60 nm) andselected harmonics. The grating period T is selected to causediffracting, out of an optical path, an incident radiation within thisband around of the design filter wavelength. Finally, the portion of theetch stop layer 330 that is exposed is removed. Alternatively, the etchstop layer 330 may be left on the first multi-layer coating if it isthin and transparent. The grating structure 360 includes a number ofridges spaced at the grating period T. Similar to the grating structure220 in FIG. 2, the ridges have a ridge width W and height H. The ridgewidth W and height H may be any suitable values. In one embodiment, theridge width W is approximately proportionally to the grating period Twith a proportionality constant α. The ridge height H is approximatelyproportionally to the grating period T with a proportionality constantβ.

[0034] Similar to the embodiment shown in FIG. 2, the reflectivespectral filter 130 in this embodiment therefore includes a firstmulti-layer coating 320, the grating structure 360, and the mirrorsubstrate 310. The grating structure 360 may include a metal spacer 340only, a second multi-layer coating 350 only, or a combination of themetal spacer 340 and the second multi-layer coating 350 as shown. Thefirst and second multi-layer coatings 320 and 340 are designed to bereflective at around the actinic wavelength (e.g., 13.4 nm) in anoptical path. With this construction, wavelengths that are both near thegrating period T and that reflect from the multi-layer coatings 320 and340 are diffracted away (diffracted rays 150 shown in FIG. 1) from thepath of the actinic rays (actinic rays 160 shown in FIG. 1).

[0035] An electromagnetic (EM) simulation is performed to study theeffects of the reflective multi-layer coating. The results of thesimulation are shown in Table 1 and FIG. 4. Table 1 shows components ofthe multi-layer coating as used in FIGS. 2 It is noted that thematerials and thickness are merely for illustrative purposes. TABLE 1Multi-layer coating components Thickness No. Layer (nm) 1 Si 3.48 2 Mo3.69 3 SiC 3.40 4 Mo 3.60 5 Si 3.41 6 Mo 3.58 7 Si 3.41 8 Mo 3.63 9 SiC3.46 10 Mo 3.53 11 Si 3.45 12 Mo 3.51 13 Si 3.47 14 Mo 3.59 15 SiC 3.4916 Mo 3.46 17 Si 3.52 18 Mo 3.44 19 Si 3.54 20 Mo 3.54 21 SiC 3.52 22 Mo3.38 23 Si 3.59 24 Mo 3.35 25 Si 3.62 26 Mo 3.49 27 SiC 3.56 28 Mo 3.2929 Si 3.66 30 Mo 3.26 31 Si 3.69 32 Mo 3.45 33 SiC 3.60 34 Mo 3.21 35 Si3.73 36 Mo 3.17 37 Si 3.77 38 Mo 3.40 39 SiC 3.64 40 Mo 3.13 41 Si 3.8042 Mo 3.10 43 Si 3.83 44 Mo 3.36 45 SiC 3.67 46 Mo 3.07 47 Si 3.86 48 Mo3.04 49 Si 3.89 50 Mo 3.33 51 SiC 3.70 52 Mo 3.02 53 Si 3.90 54 Mo 2.9955 Si 3.93 56 Mo 3.31 57 SiC 3.72 58 Mo 2.98 59 Si 3.93 60 Mo 2.95 61 Si3.96 62 Mo 3.29 63 SiC 3.74 64 Mo 2.95 65 Si 3.95 66 Mo 2.93 67 Si 3.9868 Mo 3.28 69 SiC 3.75 70 Mo 2.94 71 Si 3.97 72 Mo 2.91 73 Si 3.99 74 Mo3.27 75 SiC 3.75 76 Mo 2.92 77 Si 3.98 78 Mo 2.90 79 Si 4.00 80 Mo 3.2681 SiC 3.76 82 Mo 2.91 83 Si 3.99 84 Mo 2.89 85 Si 4.01 86 Mo 3.26 87SiC 3.76 88 Mo 2.91 89 Si 3.99 90 Mo 2.89 91 Si 4.01 92 Mo 2.6

[0036]FIG. 4 is a diagram illustrating reflectance and absorptionspectra for the multi-layer coatings according to one embodiment of theinvention. The diagram shows high reflectivity at the design wavelengthof 13.4 nm (92.5 eV) and also unwanted wavelengths longer than 60 nm(less then 20.7 eV).

[0037] The diagram shows the reflectance and absorption spectra for awhite (flat) source from a {Si (Mo SiC Mo Si Mo Si){circumflex over( )}15 Mo Si} multi-layer with thickness optimized for a 5-degreeincidence. The total number of layers is 92. The total Mo thickness is147.97 nm. The total Si thickness is 117.31 nm. The total number of SiCthickness is 54.54 nm.

[0038] It is clear from the diagram that the reflectance is maximum attwo places: a narrow band around 13.4 nm and a narrow band around 60 nm.The absorption spectrum is correspondingly minimum at these two bands.

[0039]FIG. 5 is a diagram illustrating a system 500 using a transmittingspectral filter in which one embodiment of the invention can bepracticed. The system 500 includes an EUV radiation source 510, acondenser mirror 520, a fold mirror 530, and a transmitting spectralfilter 540. The system 500 is typically used in EUV lithography.

[0040] The EUV radiation source 510 and the condenser mirror 520 are thesame as the EUV radiation source 210 and the condenser mirror 220 shownin FIG. 2. The fold mirror 530 is standard mirror that reflects theincident radiation from the condenser mirror 520. The fold mirror 530may have multi-layer coating that reflects a narrow band around a designwavelength (e.g., 13.4 nm) and wavelengths longer than a secondwavelength (e.g., 60 nm). The reflected radiation forms an optical path550.

[0041] The transmitting spectral filter 540 is inserted into the opticalpath 550. The transmitting spectral filter 540 has a grating structureor a metal mesh with a low aspect ration supported on an ultra-thinlayer of transmitting material (e.g., Nitride, Oxide). The desiredactinic rays (e.g., the 13.4 nm rays) are much shorter than the periodof the mesh or the grating structure and pass through the transmittingspectral filter 540 with little blocking. The rays having longerwavelengths that are near the period of the mesh or the gratingstructure are not transmitted and undergo reflection and diffraction,forming the reflected and diffracted rays 560.

[0042]FIG. 6 is a diagram illustrating a process 600 to fabricate thetransmitting spectral filter 540 shown in FIG. 5 according to oneembodiment of the invention.

[0043] The process 600 starts with polishing a wafer blank 610. Then, anultra-thin film layer 620 is deposited on the wafer blank 610. Theultra-thin film layer 620 is made of a transmitting material such asNitride or Oxide or any other material that has high transmittance.

[0044] Next, a metal layer 630 is deposited on the ultra-thin film layer620. Then, the metal layer 630 is lithographically patterned and etchedto become a metal mesh or a grating structure 640. The grating structure640 may have a one-dimensional layout or a two dimensional layout. Thegrating structure 640 has a grating period T responsive to rays thathave wavelengths longer than a design wavelength (e.g., 13.4 nm).Specifically, the grating structure 640 has a grating period selected toreflect or diffract incident radiation having wavelengths longer thanthe second wavelength (e.g., 60 nm). The grating structure 640 has anumber of ridges spaced at the grating period T. The ridges have a ridgewidth W and height H. The ridge width W and height H may be of anysuitable values to provide the desired characteristics. In oneembodiment, the ridge width W is approximately proportionally to thegrating period T with a proportionality constant of γ. A typical valueof γ is much less than 0.5.

[0045] Then, the process 600 lithographically patterns and etches theback side of the wafer 610 to form a support structure 615. The supportstructure 615 provides support or frame for the thin-film layer 620 andthe grating structure 640. The layout pattern 660 shows a representativetwo-dimensional layout having ridges 662, grooves 664, and supportstructure 615.

[0046] While this invention has been described with reference toillustrative embodiments, this description is not intended to beconstrued in a limiting sense. Various modifications of the illustrativeembodiments, as well as other embodiments of the invention, which areapparent to persons skilled in the art to which the invention pertainsare deemed to lie within the spirit and scope of the invention.

What is claimed is:
 1. An apparatus comprising: a grating structure on amirror substrate etched to have a grating period causing diffracting,out of an optical path, a first incident radiation within a first bandaround a first wavelength, and a multi-layer coating deposited on thegrating structure that reflects the first incident radiation, in theoptical path, within the first band and a second incident radiationwithin a second band around a second wavelength.
 2. The apparatus ofclaim 1 wherein the grating structure has one of a one-dimensionallayout and a two-dimensional layout.
 3. The apparatus of claim 1 whereinthe grating structure comprises: a plurality of ridges spaced at thegrating period, the ridges having a ridge width and height, the ridgewidth being approximately proportionally to the grating period with afirst proportionality constant, the ridge height being approximatelyproportionally to the grating period with a second proportionalityconstant.
 4. The apparatus of claim 1 wherein the first wavelength islonger than approximately 60 nm.
 5. The apparatus of claim 1 wherein thesecond wavelength is at approximately 13.4 nm.
 6. The apparatus of claim1 wherein the multi-layer coating comprises: a plurality of layers offirst and second materials having one of high and low atomic numbers,respectively, and high and low densities of charge carriers,respectively.
 7. The apparatus of claim 6 wherein the first material ismolybdenum (Mo).
 8. The apparatus of claim 6 wherein the second materialis one of silicon (Si) and beryllium (Be).
 9. The apparatus of claim 6wherein the multi-layer coating further comprises: a plurality of layersof a compound interspersed within the plurality of the first and secondmaterials.
 10. The apparatus of claim 9 wherein the compound is siliconcarbide (SiC).
 11. An apparatus comprising: a first multi-layer coatingdeposited on a mirror substrate to reflect a first incident radiation,in an optical path, within a first band around a first wavelength and asecond incident radiation within a second band around a secondwavelength; and a grating structure deposited on the first multi-layercoating, the grating structure being etched to have a grating periodcausing diffracting, out of the optical path, the second incidentradiation within the second band.
 12. The apparatus of claim 11 whereinthe grating structure comprises: a plurality of ridges spaced at thegrating period, the ridges having a ridge width and height, the ridgewidth being approximately proportionally to the grating period with afirst proportionality constant, the ridge height being approximatelyproportionally to the grating period with a second proportionalityconstant.
 13. The apparatus of claim 12 wherein each of the ridgescomprises: one of a metal spacer, a second multi-layer coating, and acombination of the metal spacer and the second multi-layer coating, themetal spacer providing grating spacing, the second multi-layer coatingreflecting the first incident radiation within the first band and thesecond incident radiation within the second band.
 14. The apparatus ofclaim 11 further comprises: a stop layer deposited between the gratingstructure and the first multi-layer coating to protect the firstmulti-layer coating during etching the grating structure.
 15. Theapparatus of claim 11 wherein the first wavelength is at approximately13.4 nm.
 16. The apparatus of claim 13 wherein each of the first andsecond multi-layer coatings comprises a plurality of layers of first andsecond materials having one of high and low atomic numbers,respectively, and high and low densities of charge carriers,respectively.
 17. The apparatus of claim 16 wherein the first materialis molybdenum (Mo).
 18. The apparatus of claim 16 wherein the secondmaterial is one of silicon (Si) and beryllium (Be).
 19. The apparatus ofclaim 16 wherein each of the first and second multi-layer coatingsfurther comprises: a plurality of layers of a compound interspersedwithin the plurality of the first and second materials.
 20. Theapparatus of claim 19 wherein the compound is silicon carbide (SiC). 21.A method comprises: etching a grating structure on a mirror substrate tohave a grating period causing diffracting, out of an optical path, afirst incident radiation within a first band around a first wavelength;and depositing a multi-layer coating on the grating structure, themulti-layer coating reflecting, in the optical path, the first incidentradiation within the first band and a second incident radiation within asecond band around a second wavelength.
 22. The method of claim 21wherein etching the grating structure comprises etching the gratingstructure to have one of a one-dimensional layout and a two-dimensionallayout.
 23. The method of claim 21 wherein etching the grating structurecomprises: etching a plurality of ridges spaced at the grating period,the ridges having a ridge width and height, the ridge width beingapproximately proportionally to the grating period with a firstproportionality constant, the ridge height being approximatelyproportionally to the grating period with a second proportionalityconstant.
 24. The method of claim 21 wherein the first wavelength islonger than approximately 60 nm.
 25. The method of claim 21 wherein thesecond wavelength is at approximately 13.4 nm.
 26. The method of claim21 wherein depositing the multi-layer coating comprises: depositing aplurality of layers of first and second materials having one of high andlow atomic numbers, respectively, and high and low densities of chargecarriers, respectively.
 27. The method of claim 26 wherein the firstmaterial is molybdenum (Mo).
 28. The method of claim 26 wherein thesecond material is one of silicon (Si) and beryllium (Be).
 29. Themethod of claim 26 wherein depositing the multi-layer coating furthercomprises: interspersing a plurality of layers of a compound within theplurality of the first and second materials.
 30. The method of claim 29wherein the compound is silicon carbide (SiC).
 31. A method comprising:depositing a first multi-layer coating on a mirror substrate, the firstmulti-layer coating reflecting, in an optical path, a first incidentradiation within a first band around a first wavelength and a secondincident radiation within a second band around a second wavelength; andetching a grating structure on the first multi-layer coating to have agrating period causing diffracting, out of the optical path, the secondradiation within the second band.
 32. The method of claim 31 whereinetching the grating structure comprises: etching a plurality of ridgesspaced at the grating period, the ridges having a ridge width andheight, the ridge width being approximately proportionally to thegrating period with a first proportionality constant, the ridge heightbeing approximately proportionally to the grating period with a secondproportionality constant.
 33. The method of claim 32 wherein etchingeach of the ridges comprises: etching one of a metal spacer, a secondmulti-layer coating, and a combination of the metal spacer and thesecond multi-layer coating, the metal spacer providing grating spacing,the second multi-layer coating reflecting the first incident radiationwithin the first band and the second incident radiation within thesecond band.
 34. The method of claim 31 further comprises: depositing astop layer between the grating structure and the first multi-layercoating to protect the first multi-layer coating during etching thegrating structure.
 35. The method of claim 31 wherein the firstwavelength is at approximately 13.4 nm.
 36. The method of claim 33wherein depositing each of the first and second multi-layer coatingscomprises depositing a plurality of layers of first and second materialshaving one of high and low atomic numbers, respectively.
 37. The methodof claim 36 wherein the first material is molybdenum (Mo).
 38. Themethod of claim 36 wherein the second material is one of silicon (Si)and beryllium (Be).
 39. The method of claim 36 wherein depositing eachof the first and second multi-layer coatings further comprises:interspersing a plurality of layers of a compound within the pluralityof the first and second materials.
 40. The method of claim 39 whereininterspersing a plurality of layers of the compound comprisesinterspersing a plurality of layers of silicon carbide (SiC).
 41. Asystem comprising: a mirror to reflect an extreme ultra violet (EUV)radiation; a baffle having an opening positioned to stop diffractedradiation rays and allowing actinic radiation rays to pass through theopening; and a reflective spectral filter positioned to generate thediffracted radiation rays and the actinic radiation rays from thereflected EUV radiation, the reflective spectral filter comprising: agrating structure on a mirror substrate etched to have a grating periodcausing diffracting, out of an optical path, a first band around a firstwavelength, and a multi-layer coating deposited on the grating structurethat reflects the first incident radiation, in an optical path, withinthe first band and a second incident radiation within a second bandaround a second wavelength.
 42. The system of claim 41 wherein thegrating structure has one of a one-dimensional layout and atwo-dimensional layout.
 43. The system of claim 41 wherein the gratingstructure comprises: a plurality of ridges spaced at the grating period,the ridges having a ridge width and height, the ridge width beingapproximately proportionally to the grating period with a firstproportionality constant, the ridge height being approximatelyproportionally to the grating period with a second proportionalityconstant.
 44. The system of claim 41 wherein the first wavelength islonger than approximately 60 nm.
 45. The system of claim 41 wherein thesecond wavelength is at approximately 13.4 nm.
 46. The system of claim41 wherein the multi-layer coating comprises: a plurality of layers offirst and second materials having one of high and low atomic numbers,respectively, and high and low densities of charge carriers,respectively.
 47. The system of claim 46 wherein the first material ismolybdenum (Mo).
 48. The system of claim 46 wherein the second materialis one of silicon (Si) and beryllium (Be).
 49. The system of claim 46wherein the multi-layer coating further comprises: a plurality of layersof a compound interspersed within the plurality of the first and secondmaterials.
 50. The system of claim 49 wherein the compound is siliconcarbide (SiC).
 51. A system comprising: a mirror to reflect an extremeultra violet (EUV) radiation; a baffle having an opening positioned tostop diffracted radiation rays and allowing actinic radiation rays topass through the opening; and a reflective spectral filter positioned togenerate the diffracted radiation rays and the actinic radiation raysfrom the reflected EUV radiation, the reflective spectral filtercomprising: a first multi-layer coating deposited on a mirror substrate,the first multi-layer coating reflecting a first incident radiation, inan optical path, within a first band around a first wavelength and asecond incident radiation within a second band around a secondwavelength, and a grating structure deposited on the first multi-layercoating, the grating structure being etched to have a grating periodcausing diffracting, out of the optical path, the second incidentradiation within the second band.
 52. The system of claim 51 wherein thegrating structure comprises: a plurality of ridges spaced at the gratingperiod, the ridges having a ridge width and height, the ridge widthbeing approximately proportionally to the grating period with a firstproportionality constant, the ridge height being approximatelyproportionally to the grating period with a second proportionalityconstant.
 53. The system of claim 52 wherein each of the ridgescomprises: one of a metal spacer, a second multi-layer coating, and acombination of the metal spacer and the second multi-layer coating, themetal spacer providing grating spacing, the second multi-layer coatingreflecting the first incident radiation within the first band and thesecond incident radiation within the second band.
 54. The system ofclaim 51 wherein the reflective spectral filter further comprises: astop layer deposited between the grating structure and the firstmulti-layer coating to protect the first multi-layer coating duringetching the grating structure.
 55. The system of claim 51 wherein thefirst wavelength is at approximately 13.4 nm.
 56. The system of claim 53wherein each of the first and second multi-layer coatings comprises aplurality of layers of first and second materials having one of high andlow atomic numbers, respectively, and high and low densities of chargecarriers, respectively.
 57. The system of claim 56 wherein the firstmaterial is molybdenum (Mo).
 58. The system of claim 56 wherein thesecond material is one of silicon (Si) and beryllium (Be).
 59. Thesystem of claim 56 wherein each of the first and second multi-layercoatings further comprises: a plurality of layers of a compoundinterspersed within the plurality of the first and second materials. 60.The system of claim 59 wherein the compound is silicon carbide (SiC).